The subject of the present invention is a novel dual-function catalytic composite, characterized by a combination of three or more metals with specified gradients on the finished catalyst particle, and its use in hydrocarbon conversion. Catalysts having both a hydrogenation-dehydrogenation function and an isomerization/cracking function ("dual-function" catalysts) are used widely in many applications, particularly in the petroleum and petrochemical industry, to accelerate a wide spectrum of hydrocarbon-conversion reactions. The isomerization/cracking function generally relates to an acid-action material of the porous, adsorptive, refractory-oxide type which is typically utilized as the support or carrier for a heavy-metal component, such as the Group VIII(IUPAC 8-10) metals, which primarily contribute the hydrogenation-dehydrogenation function. Other metals in combined or elemental form can influence one or both of the isomerization/cracking and hydrogenation-dehydrogenation functions.
In another aspect, the present invention comprehends improved processes that emanate from the use of the novel catalyst. These dual-function catalysts are used to accelerate a wide variety of hydrocarbon-conversion reactions such as dehydrogenation, hydrogenation, hydrocracking, hydrogenolysis, desulfurization, cyclization, catalytic cracking, alkylation, polymerization, and isomerization. In a specific aspect, an improved reforming process utilizes the subject catalyst to increase selectivity to gasoline and aromatics products.
Catalytic reforming comprises a variety of reaction sequences, including dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of an acyclic hydrocarbon to aromatics, hydrocracking of paraffins to light products boiling outside the gasoline range, dealkylation of alkylbenzenes and isomerization of paraffins. Some of the reactions occurring during reforming, such as hydrocracking which produces light paraffin gases, have a deleterious effect on the yield of products boiling in the gasoline range. Improvements in catalytic reforming technology thus are targeted toward enhancing those reactions effecting a higher yield of the gasoline fraction at a given octane number.
It is of critical importance that a dual-function catalyst exhibit the capability both to initially perform its specified functions efficiently and to perform them satisfactorily for prolonged periods of time. The parameters used in the art to measure how well a particular catalyst performs its intended functions in a particular hydrocarbon reaction environment are activity, selectivity and stability. In a reforming environment, these parameters are defined as follows:
Activity is a measure of the ability of the catalyst to convert hydrocarbon reactants to products at a designated severity level representing a combination of reaction conditions: temperature, pressure, contact time, and hydrogen partial pressure. Activity typically is designated as the octane number of the pentanes and heavier ("C.sub.5 +") product stream from a given feedstock at a given severity level, or conversely as the temperature required to achieve a given octane number. PA0 Selectivity refers to the percentage yield of petrochemical aromatics or C.sub.5 + gasoline product from a given feedstock at a particular activity level. PA0 Stability refers to the rate of change of activity or selectivity per unit of time or of feedstock processed. Activity stability generally is measured as the rate of change of operating temperature per unit of time/feedstock to achieve a given C.sub.5 + product octane, with a lower rate of change corresponding to better activity stability. Selectivity stability is measured as the rate of decrease of C.sub.5 + product or aromatics yield per unit of time or of feedstock.
Research and development to improve performance of reforming catalysts is being stimulated by the reformulation of gasoline, following upon widespread removal of lead antiknock additive, in order to reduce harmful vehicle emissions. Gasoline-upgrading processes such as catalytic reforming must operate at higher efficiency with greater flexibility in order to meet these changing requirements. Catalyst selectivity is becoming ever more important to tailor gasoline components to these needs while avoiding losses to lower-value products. The major problem facing workers in this area of the art, therefore, is to develop more selective catalysts while maintaining effective catalyst activity and stability.
The art teaches the use of indium in multimetallic catalysts for the catalytic reforming of naphtha feedstocks. U.S. Pat. No. 3,951,868 (Wilhelm) teaches a catalyst comprising platinum, halogen, germanium or tin, and indium wherein the ratio of indium to platinum-group metal is about 0.1:1-1:1. U.S. Pat. No. 4,522,935 (Robinson et al.) discloses a catalyst comprising a platinum-group metal, tin, indium, a halogen, and a porous support which may comprise alumina. The feature of the catalyst is an atomic ratio of indium to platinum-group metal of more than 1.35, and preferably about 2.55. Neither of these references suggest a multigradient catalyst in which the platinum-group metal is concentrated in the surface layer.
U.S. Pat. No. 4,786,625 (Imai et al.) teaches a dehydrogenation catalyst comprising a surface-impregnated platinum-group metal and a modifier selected from tin, germanium and rhenium on a refractory support having a nominal diameter of at least 850 microns. The catalyst preferably is nonacidic to minimize isomerization activity through incorporation of an alkali or alkaline earth metal.